Reference dependence of the two-determinant coupled-cluster method for triplet and open-shell singlet states of biradical molecules

Jesse J Lutz; Marcel Nooijen; Ajith Perera; Rodney J Bartlett

doi:10.1063/1.5025170

Reference dependence of the two-determinant coupled-cluster method for triplet and open-shell singlet states of biradical molecules

J Chem Phys. 2018 Apr 28;148(16):164102. doi: 10.1063/1.5025170.

Authors

Jesse J Lutz¹, Marcel Nooijen², Ajith Perera³, Rodney J Bartlett³

Affiliations

¹ Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio 45433, USA.
² Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada.
³ Quantum Theory Project, University of Florida, Gainesville, Florida 32611, USA.

PMID: 29716199
DOI: 10.1063/1.5025170

Abstract

We study the performance of the two-determinant (TD) coupled-cluster (CC) method which, unlike conventional ground-state single-reference (SR) CC methods, can, in principle, provide a naturally spin-adapted treatment of the lowest-lying open-shell singlet (OSS) and triplet electronic states. Various choices for the TD-CC reference orbitals are considered, including those generated by the multi-configurational self-consistent field method. Comparisons are made with the results of high-level SR-CC, equation-of-motion (EOM) CC, and multi-reference EOM calculations performed on a large test set of over 100 molecules with low-lying OSS states. It is shown that in cases where the EOMCC reference function is poorly described, TD-CC can provide a significantly better quantitative description of OSS total energies and OSS-triplet splittings.